Take the sentence: this is my house. In English we have my, your, his, her, our, your and their. In French they have the equivalent of each, however has a variation depending on whether the thing you are talking about (the 'house' in this example) is masculine, feminine or plural. Note that in English it is either his or her depending on who it is about, but in French this is irrelevant – it only varies based on the subject not the person.
and feminine nouns beginning
with an 'h' or a vowel
Exception: use mon instead of ma if it is followed by a noun beginning with a vowel or silent 'h'.